Twenty-two patients with the Lennox-Gastaut syndrome and their families were examined for HL-A antigens by the microlymphocytotoxicity test. The antigen HL-A7 belonging to the HL-A locus showed a significantly increased frequency (p less than 0.0005) both in parents and in patients. The same antigen showed a significantly altered segregation in patients but a normal one in healthy siblings. Another antigen of the second HL-A locus, HL-A12, did not display a normal segregation in our patients, in whom it was nearly not represented.